Sickness in complex systems
Sickness is exploiting chaotic behavior of the system in the negative way for the system in question, as it suffers and degrades in some way.
Think of a virus, just using basic biochecmistry to replicate itself to huge numbers, degrade performance of the host so much that it completely overtakes it and shits it down even.
Curing sickness is about determining which behavior is being exploited and fighting it. Entire medicine is about trying to control complex system - a human being.